The present invention relates to an explosion proof enclosed cell provided with explosion proof structure and also relates to an organic electrolytic secondary cell having such structure.
Lately, studies as to applicability of secondary cells such as lithium cells and carbon lithium cells to video tape recorders and watches are being made in various fields.
In such a cell as described above, it sometimes occurs that a chemical change in the generating element increases the internal pressure and causes an explosion. When, for example, a non-aqueous electrolytic cell such as an ordinary lithium secondary cell is put into an overcharged state by being supplied with a larger current than a normal current or put into a short-circuited condition by a misuse so that a large current is passed therethrough, it sometimes occurs that the electrolyte is decomposed to generate gas and the generated gas gradually fills up the cell, whereby internal pressure of the cell is increased and finally an explosion is caused.
To prevent such an explosion of a cell, there has been provided an explosion-proof safety device at the upper end portion of an armoring can 41, containing the generating element and serving also as the negative terminal, as shown in FIG. 1 or FIG. 2.
To be concrete, the safety device shown in FIG. 1 is attached to the topside of an insulating gasket 42 and formed of a lid plate 44 having a valve hole 43 made in the center thereof, an elastic valve body 47 made in a cylindrical form having a recess 45 formed therein with its bottom side made into a thin-walled portion 46, and a dished terminal plate 49 having a vent hole 48 made therein and arranged so as to cover the top of the elastic valve body 47, in which a cutting member 51 provided with a cutting edge 50 projecting toward the thin-walled portion 46 is disposed within the recess 45 of the elastic valve body 47.
According to the above described cell, as the generating element contained in the armoring can 41 causes a chemical change and the internal pressure of the armoring can 41 is increased, the thin-walled portion 46 of the elastic valve body 47 is expanded to move toward the cutting edge 50 provided on the cutting member 51. Then, the thin-walled portion 46 comes in abutment with the cutting edge 50, and as the internal pressure is further increased, the thin-walled portion 46 is ruptured by the cutting edge 50, whereby gas is exhausted into the air through the vent hole 48 made in the dished terminal plate 49 and explosion of the battery is prevented.
Further, the safety device shown in FIG. 2 is fitted in and supported by an insulating gasket 61 and formed of an intermediate lid 63 having a thin-walled portion 62 formed of grooves radially extended from the center and a closing lid 64 for closing the armoring can 41. Reference numeral 65 denotes a generating member constituting the generating element, which is formed of an anode material and a cathode material with separators impregnated with an electrolyte interposed therebetween and wound around a core 66 so as to form a cylinder. Reference numeral 67 denotes a lead terminal one end of which is attached to the cathode material in the cylindrically rolled form and the other end of which is led along the bottom side of an insulating plate 68, passed through a through hole 69, and attached to the bottom side of the intermediate lid 63 by welding. Reference numerals 70 and 71 denote vent holes for letting the generated gas from the generating element to outside the cell.
According to the described cell, as the gas is generated due to chemical changes in the generating element and the internal pressure of the armoring can 41 is increased, the intermediate lid 63 gradually bulges in the direction of the closing lid 64, and as the internal pressure is further increased, a rupture is caused at the thin-walled portion 62 formed in the intermediate lid 63 as shown in FIG. 3. As a result of the rupture, the gas which has been filling up the armoring can 41 is sent through the ruptured portion in the direction of the closing lid 64 and then exhausted into the air through the vent holes 70, 71, and thereby, explosion of the cell is prevented.
In the prior art explosion-proof enclosed cells as shown in FIG. 1 and FIG. 2, the increase in the internal pressure can be suppressed by the rupture of the safety valve (the elastic valve body 47 in the cell shown in FIG. 1 and the intermediate lid 63 in the cell shown in FIG. 2), but the charging current is continued to flow and the decomposition of electrolyte and active material is advanced so as to further elevate the temperature, and as a result, it sometimes occurs that the cell finally ignites. In FIG. 4 are shown changes with time of the cell voltage, charging current, and cell temperature when an overcharged state is kept up until the cell ignites (curves IV, V, and VI represent the cell voltage, charging current, and cell temperature).
The above described phenomenon occurs also in the event of short-circuiting. More particularly, if the shorting current is continued to flow even after the safety valve has been ruptured and thereby the increase in the internal pressure has been stopped, the temperature continues to rise finally causing the ignition.
By the rupture of the safety valve, such trouble can also be caused that the electrolyte leaks out of the cell through the ruptured portion and the vent hole.